It is highly desirable to provide copper alloys exhibiting a combination of high strength and high strength to ductility characteristics. It is particularly desirable to provide relatively inexpensive hot and cold workable copper alloys which exhibit high mechanical strength, favorable strength to ductility ratios and excellent formability characteristics. These copper alloys which exhibit the properties outlined above should also be convenient to process and should be able to be produced economically on a commercial scale.
Such alloys exhibiting the characteristics presented hereinabove satisfy the stringent requirements imposed by modern applications for electrical contact springs, for example, in which high strength is required coupled with good bend formability as well as resistance to mechanical property degradation at moderately elevated temperatures. This resistance to degradation is generally known as stress relaxation resistance. Commercially known copper alloys tend to exhibit deficiencies in one or more of the desirable characteristics outlined above. For example, the commercial copper Alloy 510 (a phosphor-bronze containing from 3.5 to 5.8% tin and from 0.03 to 0.35% phosphorus) exhibits superior strength properties but poor bending properties. The commercial copper Alloy 725 (a copper-nickel containing 8.5 to 10.5% nickel and from 1.8 to 2.8% tin) exhibits superior bend properties along with good solderability and contact resistance but insufficient strength properties.
One family of alloys which is able to satisfy all of the requirements presented above are the copper alloys which exhibit their combinations of properties based upon arrays of continuous, coherent precipitates in a solute depleted copper matrix, such as Cu-Ti systems containing 0.5 to 4.7% by weight Ti, the Cu-Be family of alloys containing 0.2 to 2.7% by weight Be and the various coherent precipitation reactions that can be induced to form in the various cupro-nickel compositions through the additions of third and fourth alloying elements. One example of the latter family of cupro-nickel alloys is the Cu-Ni-Al alloy system containing 5 to 30% by weight Ni and 0.5 to 5% by weight Al, in which ranges Ni.sub.3 Al forms within the alloy matrix. Another example from this particular alloy family is the Cu-Ni-Si system containing 0.5 to 15% by weight Ni and 0.5 to 3% by weight Si, in which the Ni.sub.3 Si phase, which is analogous to the Ni.sub.3 Al phase, presumably forms within the alloy matrix. A third example of the cupro-nickel alloy system may be found in the Cu-Ni-Sn system containing 3 to 30% by weight Ni and 2 to 15% by weight Sn in which a Ni-Sn rich solid soltuion precipitate forms spinodally and, therefore, continuously and coherently within the copper matrix of the alloy.
Nickel-aluminum containing copper alloys are well known in the prior art, such as disclosed in U.S. Pat. Nos. 2,101,087, 2,101,626 and 3,399,057. These patents do not contemplate the preparation of spinodal, precipitation hardened copper alloys having finely dispersed precipitates of Ni.sub.3 Al particles as disclosed in the present invention.
Thermodynamic considerations and phase equilibrium relationships dictate whether a decomposition within an alloy matrix can proceed spinodally. Spinodal decomposition is defined as a diffusion controlled, homogeneous phase separation which takes place in a solid solution whose composition and temperature is within the coherent spinodal of a miscibility gap within the two phase region of the alloy. Thus, to complete the definition of spinodal decomposition, the coherent spinodal of a miscibility gap must also be defined.
A phase diagram for a binary system, in which two solid solutions of similar crystallographic structure are in equilibrium, indicates a solid-state miscibility gap when the alloy is cooled into the two phase field so that it decomposes into the two phases. Associated with the equilibrium miscibility gap is the coherent solvus or coherent miscibility gap below which the two phases can separate coherently into the two phases. This is analogous to the situation in any two phase region where there is a coherent solvus line associated with the equilibrium solvus. Below this coherent solvus, the precipitate or second phase of the alloy system will form coherently in the matrix. The second phase forms in alignment with the crystal structure of the matrix with little distortion at the precipitate/matrix interface. Associated with this coherent solvus line is the spinodal line, below which the reaction to provide coherent precipitates via spinodal decomposition will take place.
Accordingly, it is a principal object of the present invention to provide a method for the preparation of improved copper alloys having high strength and high strength to ductility ratio characteristics.
It is a further object of the present invention to provide a method for preparing an improved copper alloy as aforesaid which has other properties such as excellent formability characteristics in the precipitation hardened condition and resistance to mechanical property degradation at moderately elevated temperatures, such as stress relaxation resistance.
It is a still further object of the present invention to provide a method for preparing an improved copper alloy as aforesaid which is convenient and economical to prepare on a commercial scale.
Additional objects and advantages will become more apparent from a consideration of the following specification.